This invention relates generally to tape drives, and more particularly to a magnetic recording tape drive with an improved system for detecting and controlling tape tension.
In tape recording systems, such as reel-to-reel type magnetic recording tape drives, the tape tension and velocity are controlled to provide a stable recording speed as well as a stable distance between the recording head and the recording surface of the tape. In low performance tape drives with relatively low accelerations to bring the tape up to the desired velocity, direct measurement of tape tension is not required. This is because a tape tension signal can be derived from the motor torque vs. current characteristics for the supply and take-up motors and from the supply and take-up reel speeds, which are inversely proportional to the diameters of the tape packs on the tape reels.
A typical magnetic recording tape drive, wherein tape tension and velocity are controlled, is shown schematically in FIG. 1. The tape drive includes a tape supply reel 15 containing tape pack 13 and driven by supply motor 16, a tape take-up reel 17 containing tape pack 14 and driven by take-up motor 18, a recording head 20 and guides, 22, 24 for guiding the tape 25 past the recording head 20. Tachometers 26a, 26b for the supply motor 16 and take-up motor 18, respectively, continually sense the supply and take-up motor speeds. A controller 30 provides output signals 23, 27 to control the motor currents to supply motor 16 and take-up motor 18, respectively. An estimator 34 is used to provide the required tape tension signal 35 and tape velocity signal 37 which are input into controller 30. The estimator 34 receives motor speed signals 31, 33 from tachometers 26a, 26b respectively, and motor current signals 23, 27 from controller 30. The estimator 34 uses the motor speed inputs (to determine the diameters of the tape packs 13, 14) and the motor current inputs (with known motor torque vs. current characteristics) to estimate the tape tension, which is fed back as signal 35 into controller 30. The estimator 34 also provides an estimate of tape velocity as signal 37 input to controller 30. A tape tension control system that estimates tape pack diameters to control motor currents is described in U.S. Pat. No. 4,408,144.
Tape drives like those depicted in FIG. 1, such as the current linear-tape-open (LTO) tape drives, do not require direct measurement of tape tension and thus do not have a separate tape tension sensor. However, future generations of type drives, especially high performance tape drives with high acceleration to bring the tape up to the desired velocity, will require direct measurement of tape tension to handle the thinner tapes that will be used. Some tape drives, like the IBM 3590, use a fixed pin tape guide in the tape path and measure tape tension directly by measuring the air pressure between the tape and the fixed pin guide. This type of direct tape tension measurement is described in U.S. Pat. No. 4,842,177. However, in tape drives such as the IBM LTO tape drive that use rollers as tape guides in the tape path instead of fixed pins, it is not possible to measure air pressure between the tape and the rollers.
Other alternatives for directly measuring tape tension have been proposed that require physical contact with the tape and/or additional components in the tape path. These include tension arms, strain gauges on roller mounts, and optical measurement of tape displacement in a flat region of the tape path. The use of an optical sensor to measure tape displacement in a flat region of the tape path is described in xe2x80x9cVacuum Puffer Head to Prevent Tape Stick on Magnetic Headxe2x80x9d, IBM Technical Disclosure Bulletin, May 1988, pp. 242-243 and xe2x80x9cTape Tension Detectionxe2x80x9d, IBM Technical Disclosure Bulletin, November 1983, pp. 2990-2991, both of which require the use of a vacuum region in the tape path, in Japanese published patent application JP-08122176A (May 17, 1996) that requires an air blower in the tape path, and in Japanese published patent application JP-2001035046A (Feb. 9, 2001) that measures tape width.
What is needed is tape drive with a tape tension sensor that does not require physical contact with the tape or additional rollers, guides or other components in the tape path.
The invention is a non-contact optical tape tension sensor for a tape drive. A light source directs an incident beam to the tape in a region of the tape path near where the tape bends. The amount of divergence of the reflected light is related to the curvature of the tape, and thus to the tape tension. The reflected light is passed through an aperture to a photodetector that detects the amount of the divergent beam passing through the aperture. The photodetector output is input to the tape drive controller that controls the currents to the supply and take-up motors to thereby maintain the tape tension within the desired range.
For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures.